How to select/list all users of an Action?
Any animation editor will show you such a list, provided the keyframed objects are selected at the moment. If you don't know which specific object contains the action you want unlinked, you can simply select everything in your scene, with a, and then check an animation editor. All keyframed objects, and ONLY keyframed objects, will be revealed in the animation editor in question. This list will show both the name of all keyframed objects, as well as the actions to which they are associated. Below, I show the view from the Dope Sheet, but it also works with the Graph Editor.